t1011: add testcase demonstrating accidental loss of user modifications

t1011: add testcase demonstrating accidental loss of user modifications

If a user has a file with local modifications that is not marked as
SKIP_WORKTREE, but the sparsity patterns are such that it should be
marked that way, and the user then invokes a command like

   * git checkout -q HEAD^

or

   * git read-tree -mu HEAD^

Then the file will be deleted along with all the users' modifications.
Add a testcase demonstrating this problem.

Note: This bug only triggers if something other than 'HEAD' is given;
if the commands above had specified 'HEAD', then the users' file would
be left alone.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>